While direct mechanical blockage of small vessels by sickled erythrocytes is undoubtedly an important factor in vaso-occlusion, there are other secondary phenomena that are likely to contribute to these episodes, including increased erythrocyte adhesion to the endothelium of post-capillary venules. SCD patients also exhibit chronic pro-coagulation changes in soluble clotting factors, as well as increased platelet number and activation. The relative contribution of these various changes to the pathophysiology of vaso-occlusive crises is unclear. One published study showed that the antiplatelet drug ticlopidine decreased frequency, duration, and severity of pain crises in SCD patients, suggesting that the increase in platelet activation does indeed contribute to vaso-occlusion.